1. Field of the Invention
Both pressure-controlled and stroke-controlled injection systems can be used to supply fuel to combustion chambers of autoignition internal combustion engines. In addition to unit injectors and unit pumps, accumulator injection systems are also used as fuel injection systems. Accumulator (common rail) injection systems advantageously make it possible to adapt the injection pressure to the load and speed of the engine. Achieving high specific outputs and reducing emissions of the autoignition engine generally require the highest injection pressure possible.
2. Prior Art
For strength reasons, the achievable pressure level in accumulator injection systems currently in use is limited to approximately 1600 bar at this time. Pressure boosters are employed to further increase the pressure in accumulator injection systems.
EP 0 562 046 B1 has disclosed an actuator/valve apparatus with damping for an electronically controlled injection unit. The actuator/valve apparatus for a hydraulic unit has an electrically excitable electromagnet with a fixed stator and a moving armature. The armature has a first and a second surface. which delimit a first and second cavity, the first surface being oriented toward the stator. A valve is provided, which is connected to the armature in a position to convey a hydraulic actuation fluid from a sump to the injection device. A damping fluid can either be collected in one of the cavities of the electromagnet device or drained from it. A region of a valve that protrudes into a central bore can selectively open and close the flow connection of the damping fluid in proportion to its viscosity.
DE 101 23 910.6 relates to a fuel injection apparatus used in an internal combustion engine. Fuel injectors supply fuel to the combustion chambers of the engine. The fuel injectors are acted on by means of a high-pressure source; the fuel injection apparatus according to DE 101 23 910.6 also includes a pressure booster that has a moving pressure booster piston, which divides a chamber that can be connected to the high-pressure source from a high-pressure chamber connected to the fuel injector. The fuel pressure in the high-pressure chamber can be varied by filling a return chamber of the pressure booster with fuel or by emptying fuel from this return chamber.
The fuel injector has a moving closing piston for opening and closing the injection openings that are oriented toward the combustion chamber. The closing piston protrudes into a closing pressure chamber so that it can be acted on with fuel pressure. This generates a force that acts on the closing piston in the closing direction. The closing pressure chamber and an additional chamber constitute a common working chamber; all of the partial regions of the working chamber are permanently connected to each other to permit the exchange of fuel.
With this design, triggering the pressure booster via the return chamber makes it possible to keep the triggering losses in the high-pressure fuel system low in comparison to a triggering by means of a working chamber that is intermittently connected to the high-pressure fuel source. In addition, the high-pressure chamber can only be pressure-relieved down to the pressure level of the high-pressure accumulator and not down to the leakage pressure level. On the one hand, this improves the hydraulic efficiency and on the other hand, permits a more rapid pressure increase up to the system pressure level so that the time intervals between injection phases can be reduced.
Pressure-controlled common rail injection systems with pressure boosters have the problem of not assuring the stability of the injection quantities to be injected into the combustion chamber, in particular the production of very small injection quantities of the kind required in preinjections. This is primarily due to the fact that the nozzle needle opens very quickly in pressure-controlled injection systems. As a result, very small variations in the triggering duration of the control valve can have powerful effects on the fuel quantity. Attempts have been made to remedy this problem by providing a separate needle stroke damper piston that delimits a damping chamber and must be guided in a high-pressure-tight clearance fit. This design does in fact permit a reduction of the needle opening speed, but on the other hand, it sharply increases the structural complexity and therefore the costs of the injection system.
In view of the ever-increasing standards regarding emissions and noise generation in autoignition internal combustion engines, further steps must be taken in the injection system in order to meet the stricter limit values to be expected in the near future.
In order to achieve the most flexible injection possible, systems with two solenoid valves have been developed. But since using two solenoid valves is complex and expensive, it would be desirable to use only one solenoid valve per injector/pressure booster combination. So far, a system of this kind has been controlled by means of a 3/2-way valve in order to produce multiple injections. These valves are complex in design and are difficult to manufacture with the required precision in series production because of the strict tolerances required.